Present-day mobile radio systems frequently operate on a packet-oriented basis. In this case, the information to be transmitted is subdivided into individual data packets. In the case of the Bluetooth Standard, which operates on a packet-oriented basis, data packets of different types and lengths are interchanged. In the case of asynchronous data traffic, a distinction can be drawn between two classes of packet types in the Bluetooth Standard. The first class of packet types comprises so-called DM packets. These have error protection by means of an FEC code (Forward Error Correction). When transmission errors occur, it is thus possible to identify errors at the receiving end, and if appropriate to correct them. In addition, the use of packet error checksums—as described below—is also envisaged.
As a second class of packet types, DH packet types have no FEC-based error protection. They in each case only have a packet error checksum at the end of the transmitted packet, from which the integrity of the received packet can be deduced at the receiving end. If a packet has errors, the receiver is requested to repeat the packet with errors. The repetition of the packet with errors is also provided for DM packet types, if complete error correction is not possible.
Furthermore, packets may also occupy two or more time slots. In the case of the Bluetooth Standard, one packet may cover one, three or five time slots. The corresponding packet types are called DM1, DM3 or DM5, respectively, for packet types with error protection coding, or DH1, DH3 and DH5, respectively, for packet types without error protection coding. A packet is in this case always transmitted on a single carrier frequency.
Finally, the Bluetooth Standard provides for the number of bits of payload information in a packet to be variable, and not to be governed exclusively by the number of time slots of the packet. The number of bits of payload information may vary between zero and a maximum number of bits that is dependent on the packet type, corresponding to a packet utilization level of 0% to 100%.
In order to optimize the data throughput of a Bluetooth connection between a transmitting appliance A and a receiving appliance B, the appliance B can determine the optimum packet type for the appliance A to use for sending the data. To do this, the appliance B has to make a statement, on the basis of measurements, about the quality of the received data, and use this to decide whether it is worthwhile changing the packet type, in terms of the coding or the number of time slots that are used. This method is referred to as CQDDR (channel quality driven data rate change), and is used in Bluetooth radio interfaces.
It is already known from the article “Negotiate Your Way to Interference-Free Bluetooth” by David McCall, from the specialist journal “Communication System Design”, July 2002 issue, pages 28 to 33, for a packet type to be selected on the basis of the bit error rate measured in the receiver when using CQDDR. Since, however, as the second class of packet types, DH packet types do not have any error protection coding, the measurement of the bit error rate is complex in this case. As an alternative, this article proposes that the selection of the suitable packet type be controlled on the basis of the ratio of confirmed or error-free packets (acknowledged packets—ACK) to unconfirmed packets (not acknowledged packets—NACK) or packets with errors. A method such as this has the disadvantage that it does not operate optimally, since the characteristics of the packets are ignored. Furthermore, long measurement time periods are required in order to obtain statistically significant statements about the packet error probability.